This invention relates in general to mobile communication devices, and more particularly to mobile communication devices capable of operating in a speakerphone mode.
Mobile communication devices, such as cellular telephones, have gained widespread use in virtually all metropolitan areas of the world. As such, manufacturers of these devices have sought to augment their usefulness by including a wide variety of features. One feature that has proven elusive is a speakerphone mode of operation. Speakerphone operation combines a relatively high power speaker and a high gain microphone with control circuitry and software so that the user can speak and hear at a distance from the speakerphone device while talking with a remote party telephonically. This is useful because the user does not have to hold the phone to the user""s ear, leaving the user""s hands free for other tasks, and it also allows others to hear and join in the conversation. One main use of speakerphones is conference calls, where several participants join in the conversation at one or both ends of the call. Speakerphones are commonly used in indoor settings in conjunction with wired telephone sets, but the wide range of acoustic environments and the small size of mobile communication devices work against satisfactory speakerphone operation. What works well for a traditional speakerphone used in an office environment produces very undesirable performance in a mobile communication device because of the much wider range of acoustic environments the device may be used in, and the physical limitation of the device.
One main aspect of speakerphone operation is the arbitration between the speaker and the microphone. In order to avoid the high audio output from feeding back into the high gain microphone, the speakerphone mutes the microphone when the far end party is speaking, and mutes the speaker when the near end party is speaking. By muting the microphone, it is meant that the signal received at the microphone is not transmitted to the remote party. This avoids echo since the remote party would otherwise hear themselves as the audio from the speaker fed back to the microphone. Some manufacturers have implemented anti-echo measures with complex digital signal processing to remove the feedback, and permit full duplex communication in speakerphone operation. However, since cost is a significant consideration in mobile communication devices, this sort of complex operation is best avoided. Furthermore, the fact that mobile communication devices are much smaller than ordinary desktop telephone sets presents a nearly insurmountable problem with regards to active echo cancellation because the high gain microphone is physically located in close proximity to the speaker. This is especially true in designs where the device has separate speakers for low and high power audio, and the high output speaker is placed near the bottom of the device, away from the earpiece speaker. In such designs, the high output speaker is extremely close to the high gain microphone.
Typically, in controlling arbitration between the speaker and the microphone, the speakerphone controller xe2x80x9clocksxe2x80x9d either the speaker or the microphone when either the remote or the near party, respectively, is speaking. Thus, if the speakerphone is receiving audio or voice signals from the remote party, and the near party is not speaking, the speakerphone will play the audio signal over the speaker, and mute the microphone to avoid feedback to the remote party (echo). Likewise, if there is no audio signal being received, and the speakerphone detects that the near party is speaking, it mutes the speaker so that any audio signals received from the remote party will not be played. In detecting speech at the near end, typically, the speakerphone simply measures the input level at the microphone, or at the audio processing circuitry, and when the level exceeds a preselected threshold, the speakerphone controller declares or decides speech is present at the microphone. If the remote party is not already speaking, the controller mutes the speaker and begins processing the audio signal at the microphone and transmitting it to the remote party. The problem with this method in a mobile communication device is that the mobile communication device may be operated in a noisy environment, and the background noise may be strong enough to lock on the microphone path and the user will not hear the remote party because the speaker is muted while the background noise overrides the controller. Therefore there is a need for a better method of detecting speech at the mobile communication device to enhance performance in a variety of acoustic environments.